By the same token, why would her knowing some other girl justify an attack?
If you've already decided to attack, one reason is as good as another. "He didn't know the square root of 169 so I beat him up."
I don't know how it was when you guys were kids, but it wasn't unknown to me that when a kid felt slighted by another, but didn't have it in them to confront them, another kid would fight on their behalf. This is how it seems to play out to me.